The fact that an equalizer made the speakers sound acceptable to you makes me think you may not be taking into account the possible effect of high-frequency hearing loss, which of course varies among individuals from non-existent to profound.
I have the same problem with my MBP 16-in 2019, whose speakers sound much boomier than previous models.
The reason is that since over the years I am progressively hearing fewer high frequencies, the ones I still do hear are drowned out by the low frequencies, which sound more pronounced to me. This is particularly noticeable with the human voice but much less so with music.
A piece of music can sound fine even though you might not be hearing all of the sounds being produced – you just won’t know which sounds you are missing – but not hearing the high frequencies of a speaking or singing voice can make it immediately difficult to understand and therefore “muddied,” “muffled” or otherwise unclear.